The present invention relates to a method of forming a wiring board and more specifically to a method of forming wiring on a printed wiring board.
The invention also relates to a wiring board obtained thereby.
Conventionally, the pattern on the conductor part (wiring part) of printed wiring boards (electronic circuit boards) has been formed by the subtractive, semi-additive and additive processes.
The subtractive process is a process in which unnecessary portions are removed from a metal layer formed on a substrate to form a pattern of a conductive part (wiring pattern), and the semi-additive and additive processes are processes in which a conductor part pattern is formed on a substrate.
In either process, the photolithographic technology has been employed to form the conductor part pattern.
The subtractive, semi-additive and additive processes are described below.
FIGS. 8A to 8F show a method of forming a pattern of a conductor part by the subtractive process.
According to the subtractive process, copper foil conductor layers 202 are formed on both surfaces of an insulating substrate 200 to obtain a copper clad laminate 204 as shown in FIG. 8A; then a through-hole 206 is formed in the copper clad laminate 204 as shown in FIG. 8B; thereafter a conductive metallic layer 208 covering the surfaces of the conductor layers 202 and the inner wall of the through-hole 206 is formed by electroless plating or electrolytic plating as shown in FIG. 8C.
Then, as shown in FIG. 8D, resist layers 210 are formed on the surfaces of the conductive metallic layer 208 on the conductor layers 202, and the resist layers 210 are selectively exposed using a photomask and developed to form the patterned resist layers 210.
Then, as shown in FIG. 8E, parts of the conductor layers 202 and the conductive metallic layer 208 which are not covered with the resist layers 210 are removed by etching, and as shown in FIG. 8F, the resist layers 210 are removed to form a printed wiring board.
Next, a method of forming a wiring pattern using the semi-additive process is shown in FIGS. 9A to 9G.
According to the semi-additive process, an insulating substrate 200 as shown in FIG. 9A is perforated to form a through-hole 206 as shown in FIG. 9B. Then, as shown in FIG. 9C, a conductive metallic layer 208 is formed by electroless plating on the surfaces of the insulating substrate 200 and the inner wall of the through-hole 206.
Then, as shown in FIG. 9D, resist layers 210 are formed on the surfaces of the conductive metallic layer 208, and the resist layers 210 are selectively exposed using a photomask and developed to form the patterned resist layers 210.
Following the patterning of the resist layers 210, a conductive metal 212 is formed by electrolytic plating as shown in FIG. 9E on part of the conductive metallic layer 208 which is not covered with the resist layers 210 and serves as a seed layer. Then, the resist layers 210 are removed as shown in FIG. 9F, and parts of the conductive metallic layer 208 which are not covered with the conductive metal 212 are removed as shown in FIG. 9G to form a printed wiring board.
Next, a method of forming a wiring pattern using the additive process is shown in FIGS. 10A to 10E.
According to the additive process, an insulating substrate (insulating layer) 200 as shown in FIG. 10A is perforated to form a through-hole 206 as shown in FIG. 10B. Then, as shown in FIG. 10C, resist layers 210 are formed on the surfaces of the insulating substrate 200, and the resist layers 210 are selectively exposed using a photomask and developed to form the patterned resist layers 210.
Following the formation of the resist layers 210, as shown in FIG. 10D, a conductive metallic layer 208 is formed by electroless plating on parts of the insulating substrate 200 which are not covered with the resist layers 210 and on the inner wall of the through-hole 206. Finally, as shown in FIG. 10E, the resist layers 210 are removed to form a printed wiring board.
The above-described method of forming the desired wiring pattern using the resist pattern formed by the photolithographic technology requires preparation of the photomask and in addition to the etching step for removing the unwanted part of the wiring metal, the step of exposing and developing the resist is also required to form the resist pattern. Hence, it takes time and cost to form the wiring pattern.
A method has recently been proposed in which the pattern of the wiring part of a printed wiring board (electronic circuit board) is formed by using a conductive fine particle dispersed ink system.
In the drawing system using conductive fine particle dispersed ink, a desired wiring pattern is formed by directly patterning a conductive, fine particulate material on the substrate in accordance with the wiring pattern using the ink-jet printing system.
Since this drawing system using conductive fine particle dispersed ink has no need to prepare a mask, it involves fewer steps than the method in which the photolithographic technology is applied to form a resist pattern to thereby form a desired wiring pattern.
However, to ensure that the fine particulate material exhibits its conductivity, baking must be performed under elevated temperatures for a prolonged period of time; this not only limits the type of substrate material on which a wiring pattern can be formed but it also increases the running cost and the size of the apparatus for performing treatment at high temperatures.
In order to solve these problems, for example, JP 2004 207558 A discloses a technique capable of forming wiring by applying for wiring drawing a colloidal metal solution to a substrate having a receptive layer formed thereon and sintering the substrate at a low temperature.
However, the inventor of the invention has made an attempt to form wiring by the method disclosed in JP 2004-207558 A and found that, although this method is an excellent technique in that it is capable of sintering at a low temperature and is also applicable to substrates including general-purpose resin films having low heat resistance, dendrite migration may occur on the periphery of the conductor part during use of the wiring board under high-temperature and high-humidity conditions to cause short circuit between mutually adjacent conductors (electrodes) just after application of current.
Migration as used herein refers to a phenomenon in which when a voltage is applied between conductors (electrodes) of a printed wiring board whose insulating properties are essentially kept at a good level and the printed wiring board is allowed to stand under high-temperature and high-humidity conditions, ions may leach out electrochemically from the conductors on or inside the insulator to lower the insulating properties between the conductors, thus causing an insulation failure or other defects of the printed wiring board.
In other words, an electrochemical reaction causes a metal on the anode of a printed wiring board to dissolve to have an ionic form and leach out to the cathode side, where the metallic ion receives an electron to be reduced and precipitated in a dendrite form from the cathode side on the electrode, thus causing migration.
Exemplary metals that may be used for the conductor include silver, lead, copper, tin, and gold, and migration easily occurs in the order of silver, lead, copper, tin, and gold. During use of silver for the conductor, migration easily occurs especially in cases where a nonconductor is not formed over the silver surface or silver is readily dissolved by the formation of a silver halide.